High-loss magnetic core for high-frequency coils



July 24, 1951 R. L. HARVEY 2,561,495

HIGH-Loss MAGNETIC coRE FOR HIGH-FREQUENCY coILs Filed Aug. 26, 1947 i l@ .l i? ffrhtZLfar@ggf ATTORNEY Patented July 24, 195i UNIT MGH-LOSS MGNEEEC CURE FR 61E-FREQUENCY @URLS Robert El. Harvey,

Princeton, N. .,l., assigner to Radio Corporation oi America, a corporation of Delaware Application August 26, 319437, Serial No. 770,729

2 Claims.

This invention relates to cores for radio-frequency coils and particularly to high-loss cores for eecting switchless band-shifting in multiband circuits.

It is an object of the invention to provide a high-loss core having such ratio of resistance to permeability that its inclusion in the magnetic eld of a high-frequency coil so reduces the Q of the coil that its resonance eiect is inappreciable.

It is a further object of the invention so to predetermine the resistance and permeability of the core for a selected frequency band that the Q of the associated coil remains substantially constant over that band or increases or decreases at desired rate with increase of frequency.

In accordance with the invention, a core is made by pressing a mass or' finely divided metal, preferably iron or equivalent magnetic material, into the desired shape and then subjecting the core to a sintering temperature for a predetermined time. By selection of the material and its neness, of the pre-sintering pressure, and of the sintering period and temperature, any ci many core-loss/frequency characteristics may be predetermined and reproduced.

The invention further resides in the highloss cores and in the methods of making them hereinafter described and claimed.

For a more detailed understanding of the invention, reference is made to the accompanying drawings, in which:

Figs. 1 to 3 disclose characteristic curves reierred to in discussion of the invention;

Figs. 4 to 6 are partial schematic diagrams o! multi-band circuits utilizing the invention; and

Figs. '7 and 8 are explanatory iigures referred to in discussion of damping cores and their circuit equivalents.

sired to operate the receiver, transmitter or the like, in a frequency-band corresponding with a different coil. For frequencies substantially l (Cil. ll7il-24l2) ing circuit without materially affecting the resonant frequency of that circuit.

Referring to Fig. l by way of example, the curve l0, based on actual measurements, shows that the Q of a coil in which a core of cold rolled steel 1s inserted rapidly increases with increasing frequency and more and more ap proaches the normal high Q of the coil itself. Apparently as the frequency increases, there is increasingly less penetration of the magnetic iieid into the core or slug and the core losses consequently rapidly decrease with increase er frequency. A core made of cold rolled steel. drill rod, or the like, though satisfactory for use as a damping core at frequencies of the order of the standard broadcast frequencies or lower. is not suited for that purpose at substantially higher frequencies: and when the coil is to be comprised in a circuit tunable over a wide range, the increasing Q of the coil at the higher and higher frequencies introduces objectionable reactive eects. Specifically, as shown in Fig. i. although the Q of the coil is about 5 at a i1'e quency of about 1.8 megacycles, at a much higher frequency the Q of the coil is increased by a very substantial factor.

The use of non-magnetic metal or dielectric cores also has proven to be even more unsatisfactory because the electromagnetic iiux or" 'the coil is apparently repelled, or at least not enough of it penetrates the core to produce losses high enough to aiiord the desired damping.

in a damping core the stored energy per cycle varies as pH", Whereas the dissipated energy per cycle varies as #QHZ T wherein mum damping or maximum reduction of Q of the coil should be obtained when #f is made as small as possible of the dissipated energy to 3 so that the optimum damping effect is in fact obtained at a compromise value which is substantially higher than that based solely on the first relation above discussed.

Lt was found that by compressing a mass of ,iinely divided iron, or like magnetic material, free of any binding material, into the desired core form and then sintering it at a temperature well below the melting point, there could be produced a core ail'ording the desired high losses even at very high frequencies; and moreover, it was found that by suitable selection of the sintering temperature and the duration of its application to the core, thereA could be obtained s loss/frequency characteristic which remained constant over a wide band of frequencies, or which slowly increased with increase of frequency, or which slowly decreased with increase of frequency.

rei'erably and by way of example, the core may be made of sponge iron powder whose particles can pass through a screen having a flneness of, for example, about 300 mesh and in any event not coarser than about 100 mesh and which are of random size and irregular shape. Apparently when a mass of such powder is sub- Jected to high pressure usually not above 100,000 pounds per square inch as for example of the order of 50,000 pounds per square inch, the particles firmly interlock due to their irregularities so that the core retains its form after removal of pressure. The core so formed is then sintered at a temperature and for a time which markedly affect its resistive and magnetic properties, particularly at the higher frequencies.

By way of example, the Q/frequency curve II of Fig. l is produced by a compressed powdered iron core which was subjected to a sintering temperature of 800 degrees C. for twenty minutes. As apparent from the curve, the normally high Q of the associated coil was reduced to about 6 at a frequency of 1.8 megacycles and progressively decreased to a Q of about 3 at a frequency of about 118 megacycles. The high-loss core so formed is therefore well suited to dampen a coil to prevent it from exhibiting any substantial reactive or resonant effect over this wide frequency range.

The curve I2 of Fig. 1 resulted from insertion in the same coil of a core similar to that produced in curve II except that the sintering temperature of 800 degrees C. was applied for a period of about forty-flve minutes. As shown by the curve, this core afforded a Q which was about 3 at a frequency of about 1.8 megacycles and slowly increased with increase of frequency, attaining a value of about 10 at a frequency of 118 megacycles. By comparison of the curves I and I2, it is evident that the powdered iron core so sintered affords substantially higher losses than those obtained by use of a solid magnetic core.

By inserting in the same coil a core of the same physical shape and dimensions which produced the curves II and I2 but which was sintered at a temperature of 800 degrees C. for thirty minutes, there was produced a high-loss core affordir the Q/frequency characteristic shown by curve Il. As evident from Fig. 1, the high losses of this core remain substantially constant over the entire frequency range of from 1.8 megacycles to 118 megacycles and in fact well beyond 4 these limits and the core is therefore ideally suited for use as a damping core.

Neither the temperature nor the time is individually critical, but if the temperature is materially decreased, the period of its application must be increased, and vice versa, to obtain the same desired resistance (r) of the core. In brief, the resistance (r) may be decreased by increasing the molding pressure, by changing the fineness of the core powder, by increasing the sintering temperature or by increasing the sintering time. The permeability a may be varied by selection of the iron. Thus a desired may be obtained by suitable choice of any one of many combinations of pressure, temperature, time and materials so to predetermine the ratio of the stored magnetic energy of the core to the dissipated energy.

The temperature of 800 degrees C. provides for a suitably short sintering time, sumcient, however, for accurate control of the desired magnetic characteristics. If the sintering temperature is substantially increased above 800 degrees C., as for example to 1000 degrees C., the differ-- ence between the periods affording the dierent magnetic characteristics exemplified by curves II, I2, i3 of Fig. 1 becomes so slight that obtainment of a particular desired damping characteristie is difilcuit to control. If, on the other hand, the temperature is reduced materially below 800 degrees C., for example to 500 degrees C., the sintering time is unnecessarily prolonged.

In rsum, the damping effect can be controlled or predetermined within wide limits by choice of the particle size of the powder, of the pressure applied in formation of the core, of the firing or sintering time. and of the sintering temperature. As these variables are interdependent, the valuev of any one or more of them may be chosen in view of other considerations such as availability, cost, or ease of control. From the general rules above and the specific examples herein given, those skilled in the art may readily produce and reproduce cores amording the desired damping characteristics.

As shown by curve I4 of Fig. 2, as a coldrolled steel core approaches the open end of a coil normally having a Q of about at a frequency of about 1.8 megacycles. the Q remains high and constant until the core is within a very short distance of the coil and then for continued movement of the core toward and into the coil, the Q rapidly falls to a low value which remains substantially constant as the core is further and further inserted. With the sintered powderediron core of the invention replacing the steel core, the resulting characteristic is shown by curve I5. As evident from Fig. 2, the curves I4 and I5 are generally quite similar at this relatively low frequency, but the sintered iron core affords a somewhat more rapid change of the Q at the beginning of its introduction into the coil and aii'ords a somewhat lower Q throughout the range of its movement within the coil.

However, at materially higher frequencies, the Q characteristics of the coil when the cores are respectively of cold-rolled steel and sintered powdered iron are markedly different. Referring to curve I0 of Fig. 3, when the core is of cold-rolled steel and the applied frequency is about 118 megacycles the Q of the coil falls com- 15 paratively slowly as the core is inserted and atpowdered iron core into the same coil affects a' much more rapid change of the Q of the coil, and a low Q oi about is attained for only -relatively slight insertion of the core.

The curves of Figs. 2 and 3 are based on measurements using cold-rolled steel and sintered powdered iron cores each having a length of one inch and a diameter of 0.246 inch. The coil used for Fig. 2 was one inch long, had an internal diameter of one-quarter inch and was wound of 220 tunis o'f #37 wire: the coil was shunted with a 100 micro-microfarad tuning condenser. The coil used for the measurements of Fig. 3 was one inch long, one-quarter inch internal diameter and was wound of 5 turns of #18 wire. The shunt capacity in this case was 13 micro-microfarads.

As apparent from curves I5-'I1 of Figs 2 and 3 and curves Il to I3 of Fig. 1, a high-loss core of sintered powdered iron is capable of reducing to about the same very low value the Q of different coils operating at frequencies widely separated in the frequency spectrum. This characteristic, obtainable as above set forth in discussion of curve I3 of Fig. 11. is of particular value in switchless band-shifting arrangements such as disclosed in the aforesaid Carlson application and especially as when one and the same highloss core is successively usedwith different coils corresponding to bands widely separated in the frequency spectrum. Referring, for example, to the multi-band amplifier circuit shown in Fig.`4, the coil I8 maybe tunable by the low-loss core I9 through the range of standard broadcast frequencies, that is, from about 0.54 megacycle tc about 1.6 megacycles and the coil l2l) may be tunable by the low-loss core 2I through a substantially higher range of frequencies, for example, from 88 megacycles to 108 megacycles. The high-loss core 22, of powdered sintered iron, is inserted in the low band coil I8 when it is desired to tune through the high band, and is inserted. in the high band coil 20 when it is desired to tune the low band. As indicated, the cores may be mechanically coupled for movement in unison so that throughout the range of movement for which the low-loss core 2I is effective to tune the coil 20 for operation in the high band, the high-loss core 22 is disposed within the coil I8 to maintain its Q, generally as discussed in connection with Fig. 2, at such low value that it exhibits-no appreciable reactive effects. Conversely, when the core assembly is so moved that the low-loss core I9 is within coil I8 for tuning in the low band, the high-loss core 22 is movable within the high band coil 20 and throughout this range of movement is effective to maintain a very low and substantially constant Q of the coil 20 so that it does not affect the tuning. As evident from curve I6 of Fig. 3, the core 22 would not be satisfactory if of cold-rolled steel because with the core I9 in coil I8 for tuning in the low band, the coil 20, despite insertion of a cold-rolled steel core 22 therein, would have a substantial Q and the circuit would respond to signals in the high band.

As shown in Fig. 5, the coil and core arrangement of Fig. 4 may be used to provide a switchless multi-band oscillator which may be used as the local oscillator of a superheterodyne receiver. or for any other purpose. In the particular oscillator circuit shown, the high band coil 20A and 6 the low band coil IBA are connected in series between the grid 23 and anode 24 of an oscillator tube 25, the grid condenser 26 serving its usual function. The positive terminal B+ of a suitable source of anode current for the tube is connected to an intermediate point of the low band coil I 8A. With the cores IQA, 2IA and 22 in the position shown in Fig. 5, the coil 20A is tunable by the low-loss core 2IA through, for example. a band of ultra-high frequencies. Throughout the p range of tuning movement of the core 2IA, generation of oscillations within the band corresponding with coil I8A is effectively prevented by insertion in the coil I8A of the high-loss core 22 of powdered sintered iron. For the high band, the oscillator functions as one of the Colpitts type and the low band coil IBA serves as a l choke coil in the line to the anode current supply.

For generation of oscillations in the low" band, the cores are moved upwardly from the position shown in Fig. 5, so that the low-loss core ISA is movable within the coil IBA. Throughout the range of tuning adjustment of the core I 9A. the generation of oscillations in the high band is prevented by the presence in coil 20A of the highloss core 22. To insure that oscillations in the high band are not generated, the core 22, as evident from prior discussion, should be of powdered sintered iron, or equivalent, as otherwise the Q of the high" band coil 20A may be suilleiently high despite insertion of the core to cause concurrent generation of both high andl lowfrequency oscillations.

` In both the arrangements of Figs. 4 and 5, the high-loss cores 22 are movable in unison with the low-loss cores and it is therefore desirable that the Q of the coil corresponding with the non-selected band fall rapidly to very low magnitude as the high-loss core approaches it. This is insured for both the high and low bands when the core is of powdered sintered iron.

The use of powdered sintered iron cores for producing high lossesin multi-band circuits is not limited to arrangements using low-loss cores, either of the magnetic or non-magnetic type, to effect so-called permeability tuning. For example, as shown in Fig. 6, the low band coil IBB and the high band coil 20B may be respectively tunable by the condensers 26 and 21, whose rotors may, if desired, be coupled for adjustment in unison. When the high-loss core 22 is within the high band coil 20B, the Q of that coil is so low that resonance of the circuit is determined by condenser 26 and its associated coil I8B, whereas when the damping core 22 is moved towithin low band coil ISB, the resonance of the circuit is determined by condenser 21 and the high band coil 20B. 'Ihus the composite circuit may be tuned by condensers 26, 21 to frequencies within either of the two bands simply by shifting the position of the high-loss core 22, thus avoiding the use of coil-switches with their attendant diiculties arising from variation of contact resistance of the switches, or of their inductance which is particularly significant at ultra-high frequencies.

In the preceding discussion, the damping effect of high-loss cores has been discussed in terms of the ratio between the energies respectively stored in and dissipated by the core and the damping effectiveness of the powdered cores of this invention has been correlated to factors controlling the effective resistance and permeability of the core. In supplement to that discussion, or as a different approach helpful to a better understanding of vrelations involved, there is now discussed the circuit equivalent of a coil having a damping coil in its magnetic field.

v The damping core and its coil may be considered the equivalent of two coupled circuits, Fig. 7, in which the coil L3 and resistor Ri correspond respectively with the inductance and resistance of the core and the coils Li and La correspond respectively with the leakage inductance of the coil and with that fraction of its total inductance which has unity or 100 per cent coupling with the inductance La of the core. The capacitor C represents the tuning capacity across the coil to be damped.

In further simplification; as the coupling between Lz and La is 100 per cent, they may, as in Fig. 8, be replaced by an equivalent inductance L directly shunted by a resistor R electrically equivalent to the resistance R1 of the core. With solid magnetic cores, the leakage inductance L1 of the coil increases with increase of frequency and so reduces the damping effect of resistance R even if the latter remained of constant value; however, with increase of frequency, the effective magnitude of resistance R decreases because of decreased penetration of the core. These effects are cumulative in effecting decreased core losses at the higher and higher frequencies with the result, as exemplified by curvey I of Fig. 1, that the Q of the coil increases at undesirably high rate with increase of frequency.

On the contrary, with the sintered powdered iron cores of this invention, the leakage inductance L1 of the coil is small throughout an extremely wide range of frequencies so that core resistance R may be considered as directly connected across the entire coil. Moreover with cores of such nature, the tendency of the eddy currents more and more to concentrate at the core surface with increase of frequency may be materially reduced, balanced or over-compensated so that, as exemplified by curves I2, I3 and I l of Fig. 1, the Q of the coil is low throughout a very wide frequency range and either slowly increases or decreases with increase of frequency, or remains constant as predetermined by selection of the conditions of the core-forming process.

For a limiting condition of Q=1 at which the circuit exhibits no selectivity, the inductive reactance (21rfL) in ohms is numerically equal to the effective alternating current resistance (R) in ohms. Under this circumstance, the yphase angle between the current in the coil and the current in the core is 45 degrees and the effective power factor of the coil is unity. With sintered powdered iron cores, this limit can be closely approached even at higher radio-frequencies for which other so-called high-loss cores of solid metal or solid dielectric are utterly ineffective.

Preferably in coil and damping core arrangevments such as exemplified by Figs. 4 to 6, the

high-loss core is disposed within the coil because the magnetic field is there the most intense. However, the core may be in the form of a case or cylinder closely fitting over the coil or may include both insertable and encasing members for the coil. Reverting to the plunger type of core, it need not be of continuous cross-section when the desired high-losses are attainable for skin depths which are less than the core radius or equivalent dimension: that is, in the case of cores having circular cross-section, the core may be a sleeve or tube whose thickness is about equal to the depth of penetration of the particular core at frequencies for which effective damping is desired.

It shall be understood that the invention is not limited to the specific methods and arrangements described and that changes and modifications may be made Within the scope of the appended claims.

What is claimed is:

1. In combination, multi-band frequency-responsive signal circuits comprising a plurality of movable low-loss cores and coils corresponding to different frequency response bands for said circuits, a ferromagnetic core of powdered magnetic material subjected to a high pressure to form a suitable core shape and subsequently sintered at a temperature well below the melting point until a loss-versus-frequency characteristic is obtained in the core which is effective to dampen a high frequency inductor to prevent it from exhibiting any substantial reactive or resonant effect over a wide frequency range, means for progressively moving said ferromagnetic core into and moving one of said low-loss cores out of the magnetic field of one of said coils in unison, so that the tuning of one of said circuits is respectively damped and controlled in frequency response over a relatively wide frequency range.

2. In combination, a high frequency circuit, a tuning coil in said circuit, a ferromagnetic core movable for insertion in said coil, said core being constructed of powdered magnetic material subjected to a high pressure to form a core shape and subsequently sintered at a temperature well below the melting point until a loss-versus-frequency characteristic is obtained in the core which is effective to dampen said coil to prevent it from exhibiting any substantial reactive or resonant effect over a wide frequency range, a low-loss core movable for insertion in said coil to tune it through a predetermined frequency band, and means mechanically coupling said cores for unitary control and selective insertion in said coil, whereby said coil is effectively switched into and out of circuit when said ferromagnetic core is moved into and out of the field of said core.

ROBERT L. HARVEY..

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Powder Metallurgy, published in Metals and Alloys, June 1942, pages 1025, 1026, 1028 and 1030. 

